they can, but they don't. Part of their argument seems to be that mobile is now a general purpose computer like Windows so it should open up. The big 3 consoles don't try to pretend that you can make host much more than games and movies on their devices, and the consoles operate on much thinner margins than the mobile market.
You can say it's cynically about money and that Google/Apple are the biggest targets. You can optimistically say it's that Epic knows consoles do more to deserve their 30% and aren't just gatekeepers (there is a LOT more support for AAA devs from the big 3 consoles than anything Google/Apple offers. Including direct help in porting if you strike deals).
We're seeing more handheld PC's, but the days of the Vita or 3DS as dedicated handheld consoles seem to be past us. There's a non-zero chance PC far in the future does the same to regular consoles too.